1. Field of the Present invention
The present invention relates to an advanced sensor architecture for use in refrigerated tractor trailer units (“reefers”). More specifically, the present invention relates to a mesh network of multiple sensors which communicate between sensors and with a processing unit of a reefer unit.
2. Description of Related Art
Heat energy can enter a closed container, like a trailer, in many ways. It can be conducted through trailer walls. It can flow around gaps in door openings or through cracked door seals. Heat can be conducted in through the trailer floor or out through the ceiling. A structural member or a steel bolt will act as a channel for heat if it passes through from outside to inside. Better reefer trailers have internal structures for rub-rails and E-tracks that are separated and insulated from the outside walls.
Other sources of heat are often the loads themselves. Exothermic foods such as chocolate ice cream, broccoli and beans will actually increase load temperature.
Mobile refrigeration units are designed to maintain, not change temperature. Reefers are not designed to chill down field loads from 100 degrees ambient temperature to 40 degrees storage temperature. They are designed to remove any increased heat and to keep product temperature stable.
Reefers also control humidity by condensing moisture from the air. To do these things, refrigeration units require four things: thermal integrity of the trailer to prevent the inflow of additional heat; sufficient BTU capacity to remove the expected amount of heat; sufficient airflow, measured in cubic feet per minute (cfm), to exchange the air inside a trailer about once every minute; and sufficient air velocity to move the air through the trailer and over and through the load.
The complex nature of reefer units creates frequent breakdowns which result in loss of products from spoilage. Recent research done by the University of Florida, in cooperation with the Florida and the U.S. Department of Agriculture, indicates that 46 percent of the drivers surveyed had loss claims, averaging one claim in a little over two years. Claims from high temperature, gases and humidity averaged more than $7,500. Yet, according to researchers, more than a third of drivers rarely or never check cargo temperatures before accepting a load, and more than half rarely or never use any recording thermometer to monitor the load in transit.
In theory, refrigerated loads come from temperature-controlled storage, spend no time on docks or in higher temperatures and the interior of a pre-cooled trailer maintains its temperature even when its doors are opened in the yard and the trailer is backed into its dock. In actuality, loads are often too warm when they are loaded, drivers don't shut their reefers off before opening doors and debris in the floor channels blocks airflow.
When a pre-cooled reefer operates with the doors open; warm, humid outside air is sucked in. The moisture hits the evaporator, which ices up. That cuts off cold airflow, while warm air fills the previously pre-cooled trailer. The same effect results when debris gets into return air passages, blocking the flow of cold air. The function of a refrigeration unit is to maintain the temperature of a load, which means removing heat from any sources, both from inside the load and outside. When cooling, the load is the warmer object, and the air surrounding the load is, or should be, cooler. That cooler air must be in contact with the load in order for it to remove heat energy. If the air does not flow then the temperature of the air in reefer unit will quickly reach the same temperature as the load. As the load warms up, so will the air. Then no cooling, not even temperature maintenance, will occur.
During the entire process of heating and cooling, the interior of the reefer is not actively monitored or managed. Further, all of the items within the reefer are treated the same and no record is kept for any single item being transported.